The F-35 Lightning II’s transonic acceleration may not meet the requirements originally set forth for the program, a top Lockheed Martin official said.

“Based on the original spec, all three of the airplanes are challenged by that spec,” said Tom Burbage, Lockheed’s program manager for the F-35. “The cross-sectional area of the airplane with the internal weapons bays is quite a bit bigger than the airplanes we’re replacing.”

The sharp rise in wave drag at speeds between Mach 0.8 and Mach 1.2 is one of the most challenging areas for engineers to conquer. And the F-35’s relatively large cross-sectional area means, that as a simple matter of physics, the jet can’t quite match its predecessors.

“We’re dealing with the laws of physics. You have an airplane that’s a certain size, you have a wing that’s a certain size, you have an engine that’s a certain size, and that basically determines your acceleration characteristics,” Burbage said. “I think the biggest question is: are the acceleration characteristics of the airplane operationally suitable?”

A recent report by the Defense Department’s top tester, J. Michael Gilmore, says that the Navy’s F-35C model aircraft, which has larger wing and tail surfaces, is not meeting requirements for acceleration.

The report doesn’t say whether the F-35A and F-35B have hit similar snags.

Richard Aboulafia, an analyst with the Teal Group, Fairfax, Va., said that the revelation was not particularly surprising.

“It’s a strike fighter,” Aboulafia said. “It’s not an interceptor; it’s not an F-22.”

Aboulafia said it was unclear whether additional engine power could boost acceleration in the difficult transonic regime. So far, doubts about the aircraft’s aerodynamic performance haven’t diminished Lockheed’s sales prospects, he said.

The F-35 transonic acceleration specifications were written based on clean-configuration F-16 Fighting Falcon and F/A-18 Hornet fighter, Burbage said.

But unlike the Hornet or the F-16, the F-35 has the same configuration unloaded as it does loaded with weapons and fuel, Burbage said. When an F/A-18 or F-16 is encumbered with weapons, pylons and fuel tanks, those jets are robbed of much of their performance.

“What is different is that this airplane has accelerational characteristics with a combat load that no other airplane has, because we carry a combat load internally,” Burbage said, the F-22 Raptor notwithstanding.

Even fully loaded, the F-35’s performance doesn’t change from its unencumbered configuration, he said.

In the high subsonic range between Mach 0.6 to Mach 0.9 where the majority of air combat occurs, the F-35’s acceleration is better than almost anything flying.

Thus far, Lockheed has not had issues with the plane’s acceleration, Burbage said. There are top level Key Performance Parameters from which lower level detailed engineering specification are derived and Lockheed’s job is to meet as many of those specifications as possible within the laws of physics, he said. Discussions are underway about if those original specifications are relevant given the jet’s acceleration in a combat configuration, Burbage added.

U.S. Air Force Lt. Col. Eric Smith, director of operations at the 58th Fighter Squadron at Eglin Air Force Base, Fla., and F-35 test pilot, said that flying the aircraft is a thrilling experience.

“I can’t even explain the adrenaline rush you get when you light the afterburner on that thing,” Smith said. “The acceleration is much better than an F-16.”

But the F-35’s aerodynamic performance is not what makes the jet special, Smith said. The F-35 powerful sensors and data-links and how that information is fused into a single coherent and easy to use display are what will make the jet an effective warplane.

Burbage added that while the F-35 is designed as a supersonic fighter, it’s not optimized for the extremely high supersonic speeds that the Raptor was designed to operate at.

I know some critics of the Program have been questioning this specific performance-related topic for years now and doubting any pertinent references to computer models, which purportedly have shown better results to the contrary, but my question would have to be: why wasn't this, as well as all the other challenges to 'laws of physics' first detected and reported back in the late 90s during the initial JSF's X-plane demonstrations?

Perhaps the program could have been better adjusted on the front-end, so as not to screw up recapitalization requirements, acquisition decision-making and strategic interests as much on the back-end?

But but but. I thought that not having external weapons was going to be a boon for performance.

This story is why the JSF needs a 40,000 lb thrust engine, because it needs that much of a push to overcome the design's inherent drag.

On the other hand, this demonstrates the costs of stealth. If you want to have stealthy internal payload, then you're going to have a large frontal cross section. There's no way around it. In the case of the F-35, that trade off is probably going to be acceptable, because the plane operationally never intended to rely on its speed to escape troubles.

But if the F-35 was not originally 'intending' to rely on speed as a requirement, then it would seem to contradict with said original specs actually requiring such acceleration, no?

Anyway, it's likely that the 'short chubby' physics factor has something to do with the acceleration characteristics and limitations as being reported now, but now that the F-35 is apparently supposed to be replacing F-15C/D air superiority role as well (due to premature F-22 cancellation), perhaps acceleration might become a more legitimate requirement?

One of the confusions found in the article however, I admit, is that one has Tom Burbage reportedly saying that the F-35's acceleration does not match it's predecessors, while on the other hand one reads remarks by test-pilots saying the acceleration is much superior to the F-16.

Somehow, I'm thinking this is a comparison of the parasitic and wave drag of the F-35 and the parasitic/wave drag of an F-16 while both are clean. Like Sferrin said, how about we stick some bombs on the F-16 or F/A-18 and see how the pressure drag on the F-35 compares to the adjusted control group's massive increase in frontal cross sectional area. Not only that but how about the concept of area ruling going completely out the window on the legacy fighters due to having to carry bombs? This is stupid. Sure, the F-35 is a fat seal that is sure to encounter huge amounts of wave drag thanks to no area ruling in the transonic regime, but since when does any fighter have to constantly try to go supersonic except for F-22 on a few occasions? Also I'd like to see an F/A-18 try to pull 8Gs with a 4000 lb. bomb load.

WHAT?! We just concluded on another thread that the F-105 is no F-35! Both are optimized for completely different flight regimes! It's almost like saying that an F-22 is the equivalent of an F-111 because both are two engined fighters that weigh over 60,000 lbs. loaded.

The F-16 is what the F-35 is replacing in the USAF, so that's what it will be compared to. The increase in weight is purely due to the difference in materials and structures associated with Low Observability, fuel fraction/lift fan, the internal weapons bay structure, etc.

@geogen..... I think you missed the point of the entire article. Bascily put Lockheed Martin understood from the very outset that the F-35 was not going to meet that specification as it was a bogus fourth generation jet specification.

The F-35 transonic acceleration specifications were written based on clean-configuration F-16 Fighting Falcon and F/A-18 Hornet fighter, Burbage said.

But unlike the Hornet or the F-16, the F-35 has the same configuration unloaded as it does loaded with weapons and fuel, Burbage said. When an F/A-18 or F-16 is encumbered with weapons, pylons and fuel tanks, those jets are robbed of much of their performance.

“What is different is that this airplane has accelerational characteristics with a combat load that no other airplane has, because we carry a combat load internally,” Burbage said, the F-22 Raptor notwithstanding.

Even fully loaded, the F-35’s performance doesn’t change from its unencumbered configuration, he said.

Thus far, Lockheed has not had issues with the plane’s acceleration, Burbage said. There are top level Key Performance Parameters from which lower level detailed engineering specification are derived and Lockheed’s job is to meet as many of those specifications as possible within the laws of physics, he said. Discussions are underway about if those original specifications are relevant given the jet’s acceleration in a combat configuration, Burbage added.

P&W wrote:The test also demonstrated the F135 propulsion system’s ability to produce margin relative to thrust with this engine producing 28 percent more thrust than the specification requirement.

This statement from PW doesn't say if the 28% is MIL 'specification', or MAX 'specification', or any other specific 'specification'. It also didn't say if the 'specification' was the original 40K or the newer 43K that was provided by PW in the final engine design. The final F135 product makes more power and weighs less than 'specification' already.

It simply indicates the F135 has 'thrust margin' and could be capable of more thrust if required, needed, wanted, and most importantly funded. It was a demonstration to counter GE's F136 claims that their engine had 'thrust margin' there-by suggesting the PW F135 had no 'growth potential'. PW smartly countered these statements with their own 'test' of an engine in a controlled manner/test-environment that showed their engine was just as flexible for future requirements.

An F100 once saw a 'test' that had it at 37,000lbs of thrust over the PW-229's 29,000lbs. A PW-232 was offered for a while at 32K, but at 37k the same motor would have not lasted long and would have burnt too much fuel to be useful for more than a few seconds at power. Technology and time has given us the PW-229EEP; notice from it's specs that PW did NOT opt for a 20% increase of thrust, but a 50% improved TBO; IE - cheaper to own for 20-30 years or more.

So while a 50K F135 would be 'cool' to the kiddies on F-16.net; at what cost to the program?

That_Engine_Guy - I knew that the F135 got 40,000 lbs of thrust, hence why I referenced that number.

As to the point about weapons load, that is highly relevant to the strike role and that should be compared, like to like. For the strike mission the JSF should by all means have a performance advantage, which is very positive mark in the JSF's book. The JSF will be in trouble if it starts to tangle with hostile interceptors over the target area, but with stealth and a functioning DAS maneuvering should be irrelevant. (Now we know why Lockheed keeps pushing that metric, between this and the buffeting.)

The only concrete performance metric this shows is that, for interception and short range A2A missions, the F-35 will probably have less performance than 4th gen fighters. There, the parasitic drag from bombs and full gas tanks will be lower, as the fighters won't be carrying that much gas. But admittedly that is very much of a niche role, and one that was outside the JSF's original specifications.

What I bet this report will have an effect on is the suitability for the JSF as a replacement for the F-15C. In that role, having high speed, or high sprint speed, can be important and the JSF's limitations in that area may cause the USAF to look elsewhere. Note, this is not a mark against the JSF, just a consequence of the programs original conception.

Not to mention the J75-P-19W at MIL created just 17,500lbs of thrust or 26,500lbs at MAX (and weighed almost 6000lbs!)

tacf-x wrote:It's almost like saying that an F-22 is the equivalent of an F-111 because both are two engined fighters that weigh over 60,000 lbs. loaded.

[sarcasm=heavy]No comparison there, the Raptor is such a slouch, only 'MACH 2 Class'... we all know the F-111 could hit MACH 2.5; the Aardvark would simply tap it's TF30s 41,600lbs of combined power and pull away from the Raptor unscathed....[\sarcasm]

TEG

[Airplanes are] near perfect, all they lack is the ability to forgive.
— Richard Collins

[sarcasm=heavy]No comparison there, the Raptor is such a slouch, only 'MACH 2 Class'... we all know the F-111 could hit MACH 2.5; the Aardvark would simply tap it's TF30s 41,600lbs of combined power and pull away from the Raptor unscathed....[\sarcasm]

You know I always though that Mach 2.5 number for the F-111 was a typo.